Image detection circuits are widely used in digital image capture applications, such as digital photography, medical imaging, industrial imaging (process control), aerospace and military imaging, and consumer applications. For example, many consumer electronics such as camcorders, cellphone cameras, security cameras, web cameras, and toys use image detection circuits to capture an electronic representation of an image.
Image detection circuits, or imagers, convert analog signals into digital signals. Imagers often include an analog-to-digital converter (ADC) to perform this conversion. One type of conventional ADC is a flash ADC. A flash ADC is also known as a parallel ADC because it converts a single analog input voltage into a multi-bit digital output signal. For example, an analog input voltage may be converted into an 8-bit or a 10-bit digital output signal. Flash ADCs are widely used in many applications because they are relatively fast, simple, and robust. In particular, many imagers use conventional flash ADCs in applications which benefit from a relatively large bandwidth.
FIG. 1 illustrates a conventional flash ADC architecture used in conventional image sensors. The conventional flash ADC architecture includes an input line coupled to a comparator ladder. The comparator ladder includes a bank of comparators. Each of the comparators compares the input voltage signal from the input line to a voltage signal from the linear resistance ladder. The linear resistance ladder, which is typically a layer of metallic trace or an arrangement of individual resistors, produces a plurality of voltages between two voltages, V1 and V2, applied at the ends of the linear resistance ladder. For example, if V1 is 1.0 Volt and V2 is 2.0 Volts, the linear resistance ladder would generate linearly distributed voltage signals at a plurality of outputs, which are coupled to the comparators. An output in the middle of the voltage resistance ladder would be approximately 1.5 Volts, while outputs closer to V1 would be between 1.0 and 1.5 Volts, and outputs closer to V2 would be between 1.5 and 2.0 Volts. The outputs are evenly distributed along the distance of the linear resistance ladder to facilitate the linear voltage signals.
The comparator ladder receives the input from the imager and the linear voltage signals from the resistance ladder to produce a thermometer code. The thermometer code is a binary code which corresponds to the voltage level on the input line. In particular, the thermometer code outputs ones for all of the comparators which receive linear voltage signals at or below the input voltage, and outputs zeros for all of the comparators which receive linear voltage signals above the input voltage (like a mercury thermometer which has mercury at and below the temperature marking, and no mercury above the temperature marking). For example, if V1 is 1.0 Volts, V2 is 2.0 Volts, and the input voltage is 1.8 Volts, all of the comparators which receive linear voltages between 1.0 and 1.8 Volts would output ones, while the remaining comparators which receive linear voltages between 1.8 and 2.0 Volts would output zeros. The thermometer code is then input to a linear encoder which converts the thermometer code to a digital output signal such as a Gray code or a binary code.
The resolution (i.e., number of bits) of the digital output signal corresponds to the number of comparators in the comparator ladder and the distribution of the linear voltages from the linear resistance ladder. For example, a 10-bit digital output signal from a conventional flash ADC uses 1,024 (i.e., 210) comparators and a corresponding number of linear voltage signals. If the linear resistance ladder has a 1.0 milliVolt step between consecutive voltage signals, then the digital output signal would represent an analog input range of about 1.0 Volts.
However, conventional flash ADC circuits have some disadvantages. Specifically, conventional flash ADCs are relatively large (i.e., they consume a relatively large area of the die). Flash ADCs also consume a lot of power. For these reasons, flash ADCs are of limited use in mobile applications which are sensitive to size and power consumption constraints. As a tradeoff, some applications use flash ADCs that have a lower resolution (i.e., less bits in the digital output signal), which offer lower quality in the captured image.